This invention pertains to a method and apparatus for dispensing fluid materials, and more particularly to a method and apparatus for dispensing flux on a surface such as a printed circuit board. The invention is particularly applicable to dispensing a low solids flux and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be advantageously employed with other types of fluxes.
In fabricating a printed circuit board (PCB), which defines a substrate surface for a printed circuit assembly, three general types of boards are frequently encountered. The first is known as a surface mount board in which only surface mount components are used. A method of securing these components to the board is known as reflow soldering where solder paste is placed on the board, and then the surface mount components located as desired. The solder paste is cured as the board is heated to reflow the solder and complete the electrical connection. Thereafter, the board is cleaned, and if a double-sided surface mount assembly is required, the same process is applied to the opposite side of the board.
The second type of board uses through hole components. As the name implies, these electrical components have leads that extend through holes or openings in the board. The leads are soldered to complete the electrical connection.
In a mixed technology board, a combination of surface mount components and through hole components are used. The surface mount components are assembled and soldered as described above. After the surface mount components are secured to the first side of the board, the through hole components are then located on the board with the component leads extending through openings in the board for a subsequent soldering process. If a double-sided assembly is envisioned, the board is inverted and the surface mount components are applied to the second surface.
In either situation, a soldering operation is required on one surface, typically the bottom surface, of the board. One common soldering process is known as wave soldering in which molten solder is pumped into a wave form and a conveyor assembly transports the board thereover. A surface of the board contacts the peak of the wave and the solder wets the protruding component leads. The solder is drawn up into the through holes to effectively and securely establish an electrical connection. The soldered board, including surface mount components previously adhesively secured to the board, requires subsequent cleaning.
The entire soldering process is actually comprised of three separate and essential steps which are normally performed within a single machine. These steps include (i) flux application, (ii) preheating the board, and (iii) soldering. Soldering flux is generally defined as "a chemically and physically active formula which promotes wetting of a metal surface by molten solder, by removing the oxide or other surface films from the base metals and the solder. The flux also protects the surfaces from reoxidation during soldering and alters the surface tension of the molten solder and the base metal." (Institute for Interconnecting and Packaging Electronic Circuits (IPC), ANSI/IPC/SF-818, "General Requirements for Electronic Soldering Fluxes" 1988 3.1.18) As described in depth in CLEANING PRINTED WIRING ASSEMBLIES IN TODAY'S ENVIRONMENT, edited by Les Hymes, and published by Van Nostrand Reinhold, a printed circuit board must be cleaned with flux to effectively prepare the board for soldering and properly wet the components.
Four general types of fluxes are in common commercial use. Of these, rosin based fluxes are the most widely used, even though they normally require a subsequent cleaning operation to remove rosin flux residue on the board. The residue can adversely impact subsequent testing of the printed circuit board.
Another major category of fluxes includes water soluble fluxes which, as the name suggests, are designed to be cleaned in an aqueous solution. For a variety of reasons, though, this technology is not yet readily accepted.
Likewise, a third category is synthetic activated fluxes, which are decreasing in demand for a number of reasons. For example, the residues must be removed with chlorofluorocarbon (CFC) based cleaners which raises environmental concerns.
A fourth type of flux receiving greater attention in light of the environmental concerns are termed low solids fluxes. Low solids fluxes contain small amounts of solids, e.g., five weight percent or less. Use of low solids fluxes are intended to limit the amount of residue remaining on the board after soldering is completed so that subsequent cleaning operations can be eliminated.
As alluded to above, commercial cleaning operations typically employ CFC's. Studies presently indicate that use of CFC's destroy, or adversely contribute to the destruction of, earth's stratospheric ozone. Thus, elimination of subsequent cleaning operations for printed circuit boards will, in turn, address environmental concerns of ozone depletion associated with post soldering cleaning processes.
Generally speaking, fluxes commonly incorporate a solvent, vehicle, activator, surfactant and antioxidant. The solvent is the liquid carrier for the flux ingredient. An isopropanol or similar type of alcohol is often used as the solvent. The vehicle component of the flux serves as a high temperature solvent during the subsequent soldering operation. The activator, on the other hand, removes contaminants such as oxides to present a wettable surface for the soldering operation. The surfactant encourages solder wetting while the antioxidant limits reoxidation of the component leads.
Known structures and methods for applying flux to a printed circuit board are described in U.S. Pat. No. 4,821,948. These conventional techniques include liquid wave, foaming, brushing, or spraying, all of which are deemed to be deficient in one manner or another in achieving the overall goals of uniformity and effectiveness of flux application.
Focusing more particularly on low solids fluxes, three methods are commonly used. The flux can be applied as a wave in a manner analogous to the wave soldering technique. An open bath of flux is pumped into a wave form and the board surface passed into the wave crest. In addition to problems associated with flux being inadvertently placed on the top of the board, the uniformity of application and ability to precisely control the amount of flux application can be problematic.
Moreover, since the bath of flux is exposed to atmosphere, the specific gravity of the flux is subject to change. With low solids fluxes, conventional techniques of controlling specific gravity with automatic density controllers is ineffective since the low amount of solids in the composition is sensitive to slight changes of solvent.
A foam fluxer may alternately be used. This flux application technique also has an open bath of flux through which air bubbles are passed to form a foam layer. The board is passed through the foam layer to apply the flux to the desired surface. Since this method also requires an open reservoir, control of the specific gravity of the flux is a concern. The bubbling and splattering associated with the foam fluxing technique lacks the desired uniformity and precision of application, along with the potential for flux to be deposited on the upper surface of the board.
The third commonly used technique of applying low solids flux to a board is spraying. The '948 patent describes one type of high velocity spray in which the flux is ultrasonically atomized. More particularly, the flux is dispersed into tiny droplets and directed into the path of a substantially laminar air flow to allegedly provide uniform flux application. An enclosing structure collects and exhausts the vapors that result from this flux application. Other sprays in which the flux is atomized, or a rotating drum spray in which the flux is atomized by an air knife from the surface of a rotating mesh drum, are known in the art.
As the spray nozzle and board move relative to one another according to the various spray techniques, the lack of uniformity of flux application is apparent. A phenomenon known as shadowing results from this relative motion and can be briefly described as areas adjacent a protruding component that fail to receive any flux, or an insufficient amount of flux, because of the relative motion. This is oftentimes categorized as a lack of uniform application, but is itself, a more particular specie of problem associated with the spray application.
Prior spraying techniques do not lend themselves to promotion of "wicking", which results from capillary action due to surface tension. Fluid material naturally tends to adopt the configuration having the lowest surface tension, and it is observed that prior techniques do not take advantage of cohesive and adhesive forces that result in flux material being attracted to desired soldering locations.
The subject invention is deemed to provide a new apparatus and method that overcomes all of the above referred to problems and others and provides for an even, uniform, and precisely controlled application of flux to a surface.